L-cysteine producing bacterium and a method for producing l-cysteine

Title: L-cysteine producing bacterium and a method for producing l-cysteine.Abstract: The present invention describes a bacterium belonging to the family Enterobacteriaceae which has L-cysteine-producing ability and has been modified to decrease the activity of a protein encoded by the d0191 gene. This bacterium is cultured in a medium, and L-cysteine, L-cystine, derivatives thereof, or a mixture thereof is collected from the medium. ...

The present invention describes a bacterium belonging to the family Enterobacteriaceae which has L-cysteine-producing ability and has been modified to decrease the activity of a protein encoded by the d0191 gene. This bacterium is cultured in a medium, and L-cysteine, L-cystine, derivatives thereof, or a mixture thereof is collected from the medium.

This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2008-056371, filed on Mar. 6, 2008, which is incorporated in its entirety by reference. The Sequence Listing in electronic format filed herewith is also hereby incorporated by reference in its entirety (File Name: US-386_Seq_List; File Size: 95 KB; Date Created: Mar. 4, 2009).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing L-cysteine or related substances. More precisely, the present invention relates to a bacterium suitable for the production of L-cysteine or related substances and a method for producing L-cysteine or related substances utilizing such a bacterium. L-cysteine and L-cysteine-related substances are used in the fields of drugs, cosmetics, and foods.

2. Brief Description of the Related Art

Microorganisms which are able to produce L-cysteine are known, for example, a coryneform bacterium with increased intracellular serine acetyltransferase activity has been described (Japanese Patent Laid-open (Kokai) No. 2002-233384). Increasing L-cysteine-producing ability by incorporating a mutant serine acetyltransferase which is desensitized to feedback inhibition by L-cysteine has also been described (Japanese Patent Laid-open No. 11-155571, U.S. Patent Published Application No. 20050112731, U.S. Pat. No. 6,218,168).

Furthermore, microorganisms in which the ability to produce L-cysteine is enhanced by suppressing the L-cysteine decomposition system are also known, for example, coryneform bacteria or Escherichia bacteria have been reported in which the activity of cystathionine-β-lyase (U.S. Patent Published Application No. 20050112731), tryptophanase (Japanese Patent Laid-open No. 2003-169668), or O-acetylserine sulfhydrylase B (Japanese Patent Laid-open No. 2005-245311) is attenuated or deleted.

Furthermore, the ydeD gene, which encodes the YdeD protein, participates in secretion of the metabolic products of the cysteine pathway (Dabler et al., Mol. Microbiol., 36, 1101-1112 (2000)). Other techniques for enhancing L-cysteine-producing ability are known, including by increasing expression of the mar locus, emr locus, acr locus, cmr locus, mex gene, bmr gene, or qacA gene. These are all genes which encode proteins which function to secrete toxic substances out of cells (U.S. Pat. No. 5,972,663). The emrAB, emrKY, yojIH, acrEF, or cusA genes are also known (Japanese Patent Laid-open No. 2005-287333).

Furthermore, an L-cysteine-producing Escherichia coli has been reported in which the activity of the positive transcriptional control factor of the cysteine regulon encoded by the cysB gene is increased (International Patent Publication WO01/27307).

The d0191 gene is a novel gene of Pantoea ananatis which was found by the inventors of the present invention. Although presumably homologous genes to d0191 have been found in various bacteria by homology searches, the functions of all of them are unknown, and their relation with L-cysteine production is also unknown.

SUMMARY OF THE INVENTION

An aspect of the present invention is to develop a novel technique for improving bacterial L-cysteine-producing ability, and thereby provide an L-cysteine-producing bacterium, and a method for producing L-cysteine, L-cystine, their derivatives, or a mixture of these by using such a bacterium.

A novel gene has been found which encodes a protein having cysteine desulfhydrase activity in Pantoea ananatis, and it has been found that L-cysteine-producing ability of a bacterium can be enhanced by modifying the bacterium so that the activity of that protein is decreased.

It is an aspect of the present invention to provide a bacterium belonging to the family Enterobacteriaceae, which has L-cysteine-producing ability and has been modified to decrease the activity of a protein selected from the group consisting of:

(A) a protein comprising the amino acid sequence of SEQ ID NO: 2, and

(B) a protein comprising the amino acid sequence of SEQ ID NO: 2 but which includes substitutions, deletions, insertions, or additions of one or several amino acid residues, and wherein said protein comprises cysteine desulfhydrase activity.

It is a further aspect of the present invention to provide the aforementioned bacterium, wherein the activity of the protein is decreased by a method selected from the group consisting of decreasing the expression of a gene encoding the protein, and by disrupting the gene encoding the protein.

It is a further aspect of the present invention to provide the aforementioned bacterium, wherein the gene is selected from the group consisting of:

(a) a DNA comprising the nucleotide sequence of SEQ ID NO: 1, and

(b) a DNA which is able to hybridize with a sequence complementary to the nucleotide sequence of SEQ ID NO: 1, or a probe prepared from the nucleotide sequence, under stringent conditions, and encodes a protein comprising cysteine desulfhydrase activity.

It is a further aspect of the present invention to provide the aforementioned bacterium, in which serine acetyltransferase has been mutated so that feedback inhibition by L-cysteine is attenuated.

It is a further aspect of the present invention to provide the aforementioned bacterium, which is a Pantoea bacterium.

It is a further aspect of the present invention to provide the aforementioned bacterium, which is Pantoea ananatis.

It is a further aspect of the present invention to provide a method for producing an L-amino acid selected from the group consisting of L-cysteine, L-cystine, derivatives thereof, and combinations thereof, which comprises culturing the aforementioned bacterium in a medium and collecting the L-amino acid from the medium.

It is a further aspect of the present invention to provide the aforementioned method, wherein the derivative of L-cysteine is a thiazolidine derivative.

It is a further aspect of the present invention to provide a DNA encoding a protein selected from the group consisting of:

(A) a protein comprising the amino acid sequence of SEQ ID NO: 2, and

(B) a protein comprising the amino acid sequence of SEQ ID NO: 2 but which includes substitutions, deletions, insertions, or additions of one or several amino acid residues, and wherein said protein comprises cysteine desulfhydrase activity.

It is a further aspect of the present invention to provide the aforementioned DNA, which is a DNA selected from the group consisting of:

(a) a DNA comprising the nucleotide sequence of SEQ ID NO: 1, and

(b) a DNA which is able to hybridize with a sequence complementary to the nucleotide sequence of SEQ ID NO: 1, or a probe prepared from the nucleotide sequence, under stringent conditions, and encodes a protein comprising cysteine desulfhydrase activity.

According to the present invention, L-cysteine-producing ability of bacteria can be improved. Furthermore, according to the present invention, L-cysteine, L-cystine, their derivatives, or a mixture thereof can be efficiently produced.

The bacterium belongs to the family Enterobacteriaceae and is able to produce L-cysteine. The bacterium has been modified so that the activity of the protein encoded by the d0191 gene is decreased. The d0191 gene will be described herein.

The ability to produce L-cysteine means that the bacterium is able to produce and cause accumulation of L-cysteine in a medium or the bacterial cells in such an amount that the L-cysteine can be collected from the medium or cells when the bacterium is cultured in the medium. A bacterium having L-cysteine-producing ability means a bacterium which can produce and cause accumulation of a larger amount of L-cysteine in a medium as compared to a wild-type, parent, or unmodified strain. The L-cysteine is present after culture of the bacterium in an amount of 0.3 g/L or more, more preferably 0.4 g/L or more, and particularly preferably 0.5 g/L or more.

Some of the L-cysteine produced by the bacterium may change into L-cystine in the medium by the formation of a disulfide bond. Furthermore, as described later, S-sulfocysteine may be generated by the reaction of L-cysteine and thiosulfuric acid which are present in the medium (Szczepkowski T. W., Nature, vol. 182 (1958)). Furthermore, the L-cysteine that is generated in the bacterial cells may be condensed with a ketone, aldehyde, or, for example, pyruvic acid, which is also present in the cells, to produce a thiazolidine derivative via the intermediate hemithioketal (refer to Japanese Patent No. 2992010). The thiazolidine derivative and hemithioketal may exist as an equilibrated mixture. Therefore, the L-cysteine-producing ability is not limited to the ability to accumulate only L-cysteine in the medium or cells, but also includes the ability to accumulate L-cystine or derivatives thereof such as S-sulfocysteine, a thiazolidine derivative, a hemithioketal, or a mixture thereof in the medium.

The bacterium having L-cysteine-producing ability may inherently be able to produce L-cysteine, or this ability may be obtained by modifying a microorganism such as those described below by mutagenesis or a recombinant DNA technique so that the microorganism has L-cysteine-producing ability. The term L-cysteine refers to the reduced type L-cysteine, L-cystine, and derivatives such as those mentioned above or a mixture thereof, unless specifically denoted otherwise.

The bacterium is not particularly limited so long as the bacterium belongs to the family Enterobacteriaceae such as those of the genera Escherichia, Enterobacter, Pantoea, Klebsiella, Serratia, Erwinia, Salmonella, and Morganella, and has L-cysteine-producing ability. Specifically, bacteria classified into the family Enterobacteriaceae according to the taxonomy used in the NCBI (National Center for Biotechnology Information) database (http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=91347) can be used. The parent, wild-type, or unmodified strain of the family Enterobacteriaceae which can be used for the modification is, preferably, a bacterium of the genus Escherichia, Enterobacter, Pantoea, Erwinia, or Klebsiella.

Although the Escherichia bacteria are not particularly limited, specifically, those described in the work of Neidhardt et al. (Backmann B. J., 1996, Derivations and Genotypes of some mutant derivatives of Escherichia coli K-12, p. 2460-2488, Table 1, In F. D. Neidhardt (ed.), Escherichia coli and Salmonella Cellular and Molecular Biology/Second Edition, American Society for Microbiology Press, Washington, D.C.) can be used. Escherichia coli is preferable. Examples of Escherichia coli include Escherichia coli W3110 (ATCC 27325), Escherichia coli MG1655 (ATCC 47076) and so forth, which are derived from the prototype wild-type strain, K12 strain.

These strains are available from, for example, the American Type Culture Collection (Address: 12301 Parklawn Drive, Rockville, Md. 20852, P.O. Box 1549, Manassas, Va. 20108, United States of America). That is, registration numbers are given to each of the strains, and the strains can be ordered by using these registration numbers (refer to http://www.atcc.org/). The registration numbers of the strains are listed in the catalogue of the American Type Culture Collection.

Examples of the Enterobacter bacteria include Enterobacter agglomerans, Enterobacter aerogenes and so forth, and examples of the Pantoea bacteria include Pantoea ananatis. Some strains of Enterobacter agglomerans were recently reclassified into Pantoea agglomerans, Pantoea ananatis, or Pantoea stewartii on the basis of nucleotide sequence analysis of the 16S rRNA etc. A bacterium belonging to any of the genus Enterobacter or Pantoea may be used so long as it is a bacterium classified into the family Enterobacteriaceae.

In particular, Pantoea, Erwinia, and Enterobacter bacteria are classified as γ-proteobacteria, and they are taxonomically very similar to one another (J. Gen. Appl. Microbiol., 1997, 43, 355-361; Int. J. Syst. Bacteriol., 1997, 43, 1061-1067). In recent years, some bacteria belonging to the genus Enterobacter were reclassified as Pantoea agglomerans, Pantoea dispersa, or the like, on the basis of DNA-DNA hybridization experiments etc. (International Journal of Systematic Bacteriology, July 1989, 39:337-345). Furthermore, some bacteria belonging to the genus Erwinia were reclassified as Pantoea ananas or Pantoea stewartii (refer to Int. J. Syst. Bacteriol., 1993, 43:162-173).

Examples of the Enterobacter bacteria include, but are not limited to, Enterobacter agglomerans, Enterobacter aerogenes, and so forth. Specifically, the strains exemplified in European Patent Publication No. 952221 can be used.

An exemplary strain of the genus Enterobacter is the Enterobacter agglomeranses ATCC 12287 strain.

Specific examples of Pantoea ananatis strains include AJ13355, SC17, and SC17(0). The SC17 strain is a low phlegm-producing mutant strain derived from the AJ13355 strain (FERM BP-6614), and was isolated from soil in Iwata-shi, Shizuoka-ken, Japan. This strain can proliferate in a low pH medium containing L-glutamic acid and a carbon source (U.S. Pat. No. 6,596,517). The SC17(0) strain was constructed to be resistant to the λ Red gene product for performing gene disruption in Pantoea ananatis (refer to Reference Example 1).

The Pantoea ananatis AJ13355 strain was deposited at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry (currently, the National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary, Address: Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Feb. 19, 1998 and assigned an accession number of FERM P-16644. It was then converted to an international deposit under the provisions of Budapest Treaty on Jan. 11, 1999 and assigned an accession number of FERM BP-6614. This strain was identified as Enterobacter agglomerans when it was isolated and deposited, and given the private number AJ13355. However, this strain was recently reclassified as Pantoea ananatis on the basis of nucleotide sequencing of the 16S rRNA and so forth.

Examples of the Erwinia bacteria include, but are not limited to, Erwinia amylovora and Erwinia carotovora, and examples of the Klebsiella bacteria include Klebsiella planticola.

Hereinafter, methods for imparting L-cysteine-producing ability to bacteria belonging to Enterobacteriaceae, or methods for enhancing L-cysteine-producing ability of such bacteria, are described.

To impart the ability to produce L-cysteine, methods conventionally employed in the breeding of coryneform bacteria or bacteria of the genus Escherichia (see “Amino Acid Fermentation”, Gakkai Shuppan Center (Ltd.), 1st Edition, published May 30, 1986, pp. 77-100) can be used. Such methods include by acquiring the properties of an auxotrophic mutant, an analogue-resistant strain, or a metabolic regulation mutant, or by constructing a recombinant strain so that it overexpresses an L-cysteine biosynthesis enzyme. Here, in the breeding of an L-cysteine-producing bacteria, one or more of the above described properties may be imparted. The expression of L-cysteine biosynthesis enzyme(s) can be enhanced alone or in combinations of two or more. Furthermore, imparting properties such as an auxotrophic mutation, analogue resistance, or metabolic regulation mutation may be combined with the methods of enhancing the biosynthesis enzymes.

An auxotrophic mutant strain, L-cysteine analogue-resistant strain, or metabolic regulation mutant strain with the ability to produce L-cysteine can be obtained by subjecting a parent, wild-type, or unmodified strain to conventional mutatagenesis, such as exposure to X-rays or UV irradiation, or by treating with a mutagen such as N-methyl-N′-nitro-N-nitrosoguanidine, etc., then selecting those which exhibit autotrophy, analogue resistance, or a metabolic regulation mutation and which also have the ability to produce L-cysteine.

The bacterium is modified to decrease the activity of the protein encoded by the d0191 gene. This protein has cysteine desulfhydrase activity. The following proteins are known to have the cysteine desulfhydrase activity of E. coli: cystathionine-β-lyase (metC product, Japanese Patent Laid-open No. 11-155571, Chandra et al., Biochemistry, 21 (1982) 3064-3069), tryptophanase (tnaA product, Japanese Patent Laid-open No. 2003-169668, Austin Newton et al., J. Biol. Chem., 240 (1965) 1211-1218)), O-acetylserine sulfhydrylase B (cysM gene product, Japanese Patent Laid-open No. 2005-245311) and the malY gene product (Japanese Patent Laid-open No. 2005-245311). In addition to decreasing the activity of the protein encoded by the d0191 gene, the activities of these proteins may be decreased.

The L-cysteine-producing bacterium preferably has a SAT which has been mutated to be resistant to feedback inhibition. The following mutations in SAT are known to induce resistance to feedback inhibition and are derived from Escherichia coli: when the methionine residue at position 256 is replaced with a glutamate residue (Japanese Patent Laid-open No. 11-155571), when the methionine residue at position 256 is replaced with an isoleucine residue (Denk, D. and Boeck, A., J. General Microbiol., 133, 515-525 (1987)), a mutation in the region from the amino acid residue at position 97 to the amino acid residue at position 273 or a deletion of the C-terminus region from the amino acid residue at position 227 (International Patent Publication WO97/15673, U.S. Pat. No. 6,218,168), when the amino acid sequence corresponding to positions 89 to 96 of wild-type SAT contains one or more mutations (U.S. Patent Published Application No. 20050112731(A1)) and so forth. In the cysE5 gene which encodes the mutant SAT described in the examples, the Val residue and the Asp residue at positions 95 and 96 of the wild-type SAT are replaced with an Arg residue and Pro residue, respectively.

The SAT gene is not limited to the gene of Escherichia coli, but can be any gene encoding a protein having the SAT activity. An SAT isozyme of Arabidopsis thaliana and desensitized to feedback inhibition by L-cysteine is known, and the gene encoding this SAT can also be used (FEMS Microbiol. Lett., 179 (1999) 453-459).

If a gene encoding a mutant SAT is introduced into a bacterium, L-cysteine-producing ability is imparted to the bacterium. To introduce a mutant SAT gene into a bacterium, various vectors which are typically used for protein expression can be used. Examples of such vectors include pUC19, pUC18, pHSG299, pHSG399, pHSG398, RSF1010, pBR322, pACYC184, pMW219, and so forth.

In order to introduce a recombinant vector containing a SAT gene into a bacterium, methods which are typically used to transform bacteria can be used, such as the method of D. A. Morrison (Methods in Enzymology, 68, 326 (1979)), treating recipient cells with calcium chloride to increase permeability of the cells for DNA (Mandel, M. and Higa, A., J. Mol. Biol., 53, 159 (1970)), and a method based on electroporation.

Furthermore, the SAT activity can also be enhanced by increasing the copy number of the SAT gene. The copy number of the SAT gene can be increased by introducing the SAT gene into a bacterium by using a vector such as those described above, or by introducing multiple copies of the SAT gene onto the chromosomal DNA of a bacterium. Multiple copies of the SAT gene are introduced by homologous recombination which targets a sequence present on the chromosomal DNA in a multiple copies. A repetitive DNA or inverted repeat present at the end of a transposable element can be used as a sequence which is present on the chromosomal DNA in a multiple copies. Alternatively, as disclosed in Japanese Patent Laid-open No. 2-109985, multiple copies of the SAT gene can be introduced into the chromosomal DNA by incorporating them into a transposon and transferring it.

The bacterium can be obtained by modifying a Enterobacteriaceae bacterium which is able to produce cysteine so that the activity of the protein encoded by the d0191 gene (henceforth also referred to as “D0191”) is decreased. Alternatively, after modifying the bacterium so that the activity of the D0191 protein is decreased, the L-cysteine-producing ability may then be imparted.

A novel gene encoding a protein having cysteine desulfhydrase activity from the chromosomal DNA of Pantoea ananatis has been found, and has been designated the d0191 gene. The nucleotide sequence of this gene and the amino acid sequence encoded by the gene are shown in SEQ ID NOS: 1 and 2, respectively.

When databases were searched for the sequence of the d0191 gene of Pantoea ananatis, homologous genes to the d0191 gene were found in the following bacteria, although their functions were unknown. The nucleotide sequences of these genes and their encoded amino acid sequences are shown in SEQ ID NOS: 34 to 52. Accession numbers in the NCBI (National Center for Biotechnology Information) database are shown in the parentheses (GenBank Identifier(gi)|RefSeq accession (ref)).

In the present invention, the d0191 gene of Pantoea ananatis and it's homologues which are native to, or derived from, other bacteria are also encompassed by the term ‘the d0191 gene’.

The phrase “decrease the activity of the protein encoded by the d0191 gene” means that the activity of the D0191 protein encoded by the d0191 gene is decreased as compared to a non-modified strain such as a wild-type strain or parent strain, and may also mean the complete disappearance of the activity.

As described in the examples, it was demonstrated that the protein encoded by the d0191 gene, i.e., the D0191 protein, has cysteine desulfhydrase activity by staining. Therefore, the activity of the D0191 protein specifically means cysteine desulfhydrase activity. The cysteine desulfhydrase activity can be measured by, for example, the method described in Japanese Patent Laid-open No. 2002-233384.

Modifications which result in a decrease in the activity of the D0191 protein include, for example, reducing expression of the d0191 gene. Specifically, for example, intracellular activity of the protein can be reduced by deleting a part or all of the coding region of the d0191 gene on the chromosome. Furthermore, the activity of the D0191 protein can also be decreased by reducing the expression by, for example, modifying an expression control sequence of the gene such as promoter or Shine-Dalgarno (SD) sequence. Furthermore, the expression of the gene can also be reduced by modifying a non-translated region other than an expression control sequence. Furthermore, the whole gene including the sequences on both sides of the gene on the chromosome may be deleted. Furthermore, a decrease in activity can also be attained by introducing a mutation for an amino acid substitution (missense mutation), a stop codon (nonsense mutation), or a frame shift mutation which adds or deletes one or two nucleotides into the coding region of the d0191 gene on the chromosome (Journal of Biological Chemistry, 272:8611-8617 (1997); Proceedings of the National Academy of Sciences, USA, 95 5511-5515 (1998); Journal of Biological Chemistry, 266, 20833-20839 (1991)).

Furthermore, a transcriptional regulator gene which is involved in regulating the expression of the d0191 gene of the Pantoea ananatis SC17 strain may be used. This gene was named c0263. The ORF of the c0263 gene is located 80 bp upstream from the d0191 ORF in the opposite direction as the d0191. The c0263 gene is thought to be a homolog of the ybaO gene, which has been found in bacteria such as E. coli, by homology searches and encodes a translation regulator. The activity of the d0191 protein can be decreased by inactivating the ybaO gene. Additionally, both the c0263 and d0191 genes may be deleted. The nucleotide sequence of the d0191 gene from Pantoea ananatis and the amino acid sequence encoded by the gene are shown in SEQ ID NOS: 66 and 67, respectively.

Furthermore, modifications can be made by conventional mutagenesis using X-ray or ultraviolet irradiation, or via the use of a mutagen such as N-methyl-N′-nitro-N-nitrosoguanidine, so long as the modification results in a decrease in the activity of the D0191 protein.

One or more nucleotides, more preferably two or more nucleotides, particularly preferably three or more nucleotides in the expression control sequence can be modified. When deletions are made to the coding region, the deletions can be made in the N-terminus region, an internal region, or the C-terminus region, or even the entire coding region may be deleted, so long as the function of the d0191 protein is decreased or eliminated. The longer the region is that is deleted, the more likely inactivation of the gene will occur. Furthermore, it is preferred that reading frames upstream and downstream of the region to be deleted are not the same.

When another sequence is inserted into the coding region of the d0191 gene, the sequence may be inserted into any region of the gene, and the longer the sequence is that is inserted, the more likely the gene will be inactivated. It is preferred that reading frames upstream and downstream of the insertion site are not the same. The sequence that is inserted is not particularly limited so long as the function of the encoded D0191 protein is decreased or eliminated, and examples include a transposon carrying an antibiotic resistance gene, a gene useful for L-cysteine production and so forth.

The d0191 gene on the chromosome can be modified as described above, for example, by preparing a deletion-type version of the gene in which a partial sequence of the gene is deleted so that the deletion-type gene produces a non-functioning D0191 protein, and transforming a bacterium with a DNA containing the deletion-type gene to cause homologous recombination between the deletion-type gene and the native gene on the chromosome, and thereby substituting the deletion-type gene for the gene on the genome. The D0191 protein encoded by the deletion-type gene has a conformation different from that of the wild-type protein, if it is even produced, and thus the function is reduced or deleted. Gene disruption based on gene substitution utilizing homologous recombination is known, and includes the method called Red-driven integration (Datsenko, K. A, and Wanner, B. L., Proc. Natl. Acad. Sci. USA, 97:6640-6645 (2000)), use of a linear DNA in Red driven integration in combination with an excisive system derived from phage (Cho, E. H., Gumport, R. I., Gardner, J. F., J. Bacteriol., 184:5200-5203 (2002)), using a plasmid containing a temperature-sensitive replication origin or a plasmid capable of conjugative transfer, a method of utilizing a suicide vector not having replication origin in a host (U.S. Pat. No. 6,303,383, Japanese Patent Laid-open No. 05-007491), and so forth.

Decrease of the expression of the d0191 gene can be confirmed by comparing the amount of mRNA transcribed from the gene with that in a wild-type strain or non-modified strain. The expression can be confirmed by Northern hybridization, RT-PCR (Sambrook, J., Fritsch, E. F., and Maniatis, T. 1989, Molecular Cloning A Laboratory Manual/Second Edition, Cold Spring Harbor Laboratory Press, New York.), and the like.

A decrease of the amount of the D0191 protein can be confirmed by Western blotting using antibodies (Molecular cloning, Cold spring Harbor Laboratory Press, Cold spring Harbor, USA, 2001).

A decrease of the amount of the D0191 protein can also be confirmed by measuring the cysteine desulfhydrase activity in the cell.

Since the nucleotide sequence of the d0191 gene may different depending on the bacterial species or strain that is chosen, the d0191 gene to be modified may be a variant of the nucleotide sequence of SEQ ID NO: 1. Moreover, the d0191 gene to be modified may be any of the aforementioned d0191 gene homologues, for example, a variant of a gene having the nucleotide sequence shown as SEQ ID NOS: 33, 35, 37, 39, 41, 43, 45, 47, 49, or 51.

Variants of the d0191 gene can be found by using BLAST to search (http://blast.genome.jp/) by referring to the nucleotide sequence of SEQ ID NO: 1 or SEQ ID NO: 33, 35, 37, 39, 41, 43, 45, 47, 49, or 51. Moreover, variants of the d0191 gene include genes which can be amplified by PCR using the chromosome of bacterium belonging to the family Enterobacteriaceae or the like as the template, and synthetic oligonucleotides prepared on the basis of the nucleotide sequence of SEQ ID NO: 1, SEQ ID NO: 33, 35, 37, 39, 41, 43, 45, 47, 49, or 51.

Moreover, the d0191 gene may also encode a protein having a sequence corresponding to the amino acid sequence of the D0191 protein, such as the sequence of SEQ ID NO: 2, SEQ ID NO: 34, 36, 38, 40, 42, 44, 46, 48, 50 or 52, including substitutions, deletions, insertions, additions, or the like of one or several amino acid residues at one or several positions. Although the number of the “one or several” amino acid residues may differ depending on their position in the three-dimensional structure or the types of amino acid residues of the proteins, it is preferably 1 to 20, more preferably 1 to 10, particularly preferably 1 to 5. These substitutions, deletions, insertions, or additions of one or several amino acids are preferably conservative mutations so that the function of the protein in maintained. A conservative mutation is a mutation wherein substitution takes place mutually among Phe, Trp and Tyr, if the substitution site is an aromatic amino acid; among Leu, Ile and Val, if the substitution site is a hydrophobic amino acid; between Gln and Asn, if it is a polar amino acid; among Lys, Arg and His, if it is a basic amino acid; between Asp and Glu, if it is an acidic amino acid; and between Ser and Thr, if it is an amino acid having a hydroxyl group. Typical examples of conservative mutations are conservative substitutions. Specific examples of conservative substitutions include: substitution of Ser or Thr for Ala; substitution of Gln, His or Lys for Arg; substitution of Glu, Gln, Lys, His or Asp for Asn; substitution of Asn, Glu or Gln for Asp; substitution of Ser or Ala for Cys; substitution of Asn, Glu, Lys, His, Asp or Arg for Gln; substitution of Gly, Asn, Gln, Lys or Asp for Glu; substitution of Pro for Gly; substitution of Asn, Lys, Gln, Arg or Tyr for His; substitution of Leu, Met, Val or Phe for Ile; substitution of Ile, Met, Val or Phe for Leu; substitution of Asn, Glu, Gln, His or Arg for Lys; substitution of Ile, Leu, Val or Phe for Met; substitution of Trp, Tyr, Met, Ile or Leu for Phe; substitution of Thr or Ala for Ser; substitution of Ser or Ala for Thr; substitution of Phe or Tyr for Trp; substitution of His, Phe or Trp for Tyr; and substitution of Met, Ile or Leu for Val. The above-mentioned amino acid substitution, deletion, insertion, addition, inversion etc. may be the result of a naturally-occurring mutation or variation due to an individual difference, or a difference of species of a bacterium.

The d0191 gene encodes a protein having the amino acid sequence of SEQ ID NO: 2, or a protein having an amino acid sequence of SEQ ID NO: 2 which includes substitutions, deletions, insertions or additions of 1 to 50, preferably 1 to 40, more preferably 1 to 30, still more preferably 1 to 20, further still more preferably 1 to 10, particularly preferably 1 to 5, of amino acid residues, and has cysteine desulfhydrase activity.

Furthermore, the gene having such one or more conservative mutations as mentioned above may encode a protein showing a homology of 80% or more, preferably 90% or more, more preferably 95% or more, still more preferably 97% or more, further still more preferably 98% or more, particularly preferably 99% or more, to the entire encoded amino acid sequence, and which has a function equivalent to that of the wild-type D0191 protein. In the present specification, the term “homology” may mean “identity”.

The d0191 gene may be a DNA which hybridizes with a probe prepared from known gene sequences, for example, the above described gene sequences or sequences complementary to the sequences under stringent conditions and which encode a protein which is a functional equivalent to the D0191 protein. The term “stringent conditions” refers to conditions where a so-called specific hybrid is formed and a non-specific hybrid is not formed. Examples thereof include conditions where DNAs having high homology, for example, at least 80%, preferably 90%, more preferably 95%, more preferably 97%, more preferably 98%, further preferably 99% homology, hybridize with each other and DNAs having homology less than the value do not hybridize with each other; and specifically include conditions corresponding to a salt concentration and temperature of washing which are typical of Southern hybridization, e.g., washing at 60° C., 1×SSC, 0.1% SDS, preferably 60° C., 0.1×SSC, 0.1% SDS, more preferably 68° C., 0.1×SSC, 0.1% SDS, once or preferably twice or three times.

The probe may be a partial sequence of the gene. Such a probe can be prepared by PCR using oligonucleotides prepared based on the known nucleotide sequences of genes as primers, and a DNA fragment containing these sequences as the template. When a DNA fragment of a length of about 300 bp is used as the probe, the conditions of washing after hybridization can be, for example, 50° C., 2×SSC, and 0.1% SDS.

The above descriptions about variants of genes and proteins are similarly applied to enzymes such as serine acetyltransferase and genes coding for them.

These compounds can be produced by culturing the bacterium obtained as described above in a medium, and collecting L-cysteine, L-cystine, derivatives thereof, or a mixture thereof from the medium. Examples of a derivative of L-cysteine include S-sulfocysteine, a thiazolidine derivative, a hemithioketal corresponding the thiazolidine derivative mentioned above and so forth.

Examples of the medium used for the culture include ordinary media containing a carbon source, nitrogen source, sulfur source, inorganic ions, and other organic components as required.

As the carbon source, saccharides such as glucose, fructose, sucrose, molasses and starch hydrolysate, and organic acids such as fumaric acid, citric acid and succinic acid can be used.

As the nitrogen source, inorganic ammonium salts such as ammonium sulfate, ammonium chloride and ammonium phosphate, organic nitrogen such as soybean hydrolysate, ammonia gas, aqueous ammonia, and so forth can be used.

As the sulfur source, inorganic sulfur compounds, such as sulfates, sulfites, sulfides, hyposulfites, and thiosulfates can be used.

As organic trace amount nutrients, it is desirable to add required substances such as vitamin B1, yeast extract and so forth in appropriate amounts. Other than these, potassium phosphate, magnesium sulfate, iron ions, manganese ions, and so forth are added in small amounts.

The culture is preferably performed under aerobic conditions for 30 to 90 hours. The culture temperature is preferably controlled to be 25° C. to 37° C., and the pH is preferably controlled to be 5 to 8 during the culture. To adjust the pH, inorganic or organic acidic or alkaline substances, ammonia gas and so forth can be used. Collection of L-cysteine from the culture can be attained by, for example, any combination of known ion exchange resin methods, precipitation, and other known methods.

L-cysteine obtained as described above can be used to produce L-cysteine derivatives. The cysteine derivatives include methylcysteine, ethylcysteine, carbocysteine, sulfocysteine, acetylcysteine, and so forth.

Furthermore, when a thiazolidine derivative of L-cysteine is produced in the medium, L-cysteine can be produced by collecting the thiazolidine derivative from the medium to break the reaction equilibrium between the thiazolidine derivative and L-cysteine so that L-cysteine should be excessively produced.

Furthermore, when S-sulfocysteine is produced in the medium, it can be converted into L-cysteine by reduction with a reducing agent such as dithiothreitol.

EXAMPLES

Hereinafter, the present invention will be explained more specifically with reference to the following non-limiting examples.

Reference Example 1Construction of Pantoea ananatis Strain which is Resistant to the λ Red Gene Product

To disrupt the desired gene in Pantoea ananatis, a recipient strain was constructed which is able to efficiently carry out the method called “Red-driven integration” or “Red-mediated integration” (Proc. Natl. Acad. Sci. USA., 97, 6640-6645 (2000)).

First, the novel helper plasmid RSF-Red-TER was constructed. This plasmid expresses the gam, bet, and exo genes of λ (henceforth referred to as “λ Red gene”) (FIG. 1). The details are described in Reference Example 2.

This plasmid can be used in a wide range of hosts having different genetic backgrounds. This is because 1) this plasmid has the replicon of the RSF110 wide-host spectrum plasmid (Scholz, et al., 1989; Buchanan-Wollaston et al., 1987), which may be stably maintained by many gram negative and gram positive bacteria, and even plants, 2) the λ Red genes, including the gam, bet, and exo genes, are under the control of the PlacUV5 promoter, which is recognized by the RNA polymerases of many bacteria (for example, Brunschwig, E. and Darzins, A., Gene, 111, 1, 35-41 (1992); Dehio, M. et al, Gene, 215, 2, 223-229 (1998)), and 3) the autoregulation factor PlacUV5-lacI and the ρ-non-dependent transcription terminator (TrrnB) of the rrnB operon of Escherichia coli lowers the basal expression level of the λ Red gene (Skorokhodova, A. Yu et al, Biotekhnologiya (Rus), 5, 3-21 (2004)). Furthermore, the RSF-Red-TER plasmid contains the levansucrase gene (sacB), and by using this gene, the plasmid can be collected from cells in a medium containing sucrose.

In Escherichia coli, the frequency of integration of a PCR-generated DNA fragment and the short flanking region provided by the RSF-Red-TER plasmid is high, and is typically to the same extent as when the pKD46 helper plasmid is used (Datsenko, K A., Wanner, B. L., Proc. Natl. Acad. Sci. USA, 97, 6640-6645 (2000)). However, expression of the λ Red gene is toxic to Pantoea ananatis. The cells transformed with the RSF-Red-TER helper plasmid grew extremely slowly in LB medium containing IPTG (isopropyl-β-D-thiogalactopyranoside, 1 mM) and an appropriate antibiotic (25 μg/ml of chloramphenicol or 40 μg/ml of kanamycin), and the efficiency of λ Red-mediated recombination is extremely low (10−8), if observed at all.

A variant strain of Pantoea ananatis which is resistant to expression of all of the λ Red genes was selected. For this purpose, the RSF-Red-TER plasmid was introduced into the Pantoea ananatis SC17 strain (U.S. Pat. No. 6,596,517) by electroporation. After an 18 hour culture, about 106 transformants were obtained, and among these, 10 clones formed colonies of large size, and the remainder formed extremely small colonies. After an 18 hour culture, the large colonies had a size of about 2 mm, and the small colonies had a size of about 0.2 mm. Whereas the small colonies did not grow any more even if the culture was extended until another 24 hours, the large colonies continued to grow. One of the large colony Pantoea ananatis variant strains which was resistant to expression of all the λ Red genes (gam, bet, and exo) was used for the further analysis.

The RSF-Red-TER plasmid DNA was isolated from one clone of the large colony clones, and from several clones of the small colonies, and transformed again into Escherichia coli MG1655 to examine the ability of the plasmid to synthesize an active Red gene product. By a control experiment for Red-dependent integration in the obtained transformants, it was demonstrated that only the plasmid isolated from the large colony clone induced expression of the λ Red gene required for the Red-dependent integration. In order to investigate whether the Red-mediated integration occurs in the selected large colony clone, electroporation was performed by using a linear DNA fragment produced by PCR. This fragment was designed so that it contains a KmR marker and a flanking region of 40 bp homologous to the hisD gene. This fragment is integrated into the hisD gene of Pantoea ananatis at the SmaI recognition site. Two small colony clones were used as a control. The nucleotide sequence of the hisD gene of Pantoea ananatis is shown in SEQ ID NO: 3. For PCR, the oligonucleotides of SEQ ID NOS: 4 and 5 were used as primers, and the pMW118-(λatt-Kmr-λatt) plasmid was used as the template. The two small colony clones which were not resistant to the λ Red gene were used as a control. Construction of the pMW118-(λattL-Kmr-λattR) plasmid will be explained in detail in Reference Example 3.

The RSF-Red-TER plasmid can induce expression of the Red gene when the lac gene is present on the plasmid. Two kinds of induction conditions were investigated. In the first group, IPTG (1 mM) was added 1 hour before the electroporation, and in the second group, IPTG was added at the start of the culture. The growth rate of the cells harboring RSF-Red-TER derived from the large colony clone was not significantly lower than that of a strain not having the SC17 plasmid. The addition of IPTG only slightly decreased the growth rate of these cultures. On the other hand, the progeny of the small colony clones grew extremely slowly even without the addition of IPTG, and after induction, growth was substantially arrested. After electroporation of the cells of the progeny of the large colony clone, many KmR clones grew (18 clones after a short induction time, and about 100 clones after an extended induction time). All the 100 clones that were investigated had a His− phenotype, and about 20 clones were confirmed by PCR to have the expected structure of chromosome in the cells. On the other hand, even when electroporation was performed with the progeny of the small colony clones, an integrated strain was not obtained.

The large colony clone was grown on a plate containing 7% sucrose to eliminate the plasmid, and transformed again with RSF-Red-TER. The strain without the plasmid was designated SC17(0). This strain was deposited at the Russian National Collection of Industrial Microorganisms (VKPM, GNII Genetica, address: Russia, 117545 Moscow, 1 Dorozhny proezd. 1) on Sep. 21, 2005, and assigned an accession number of VKPM B-9246.

All the clones which grew after the aforementioned re-transformation showed large colony sizes like the parent strain clone SC17(0). The Red-mediated integration experiment was performed in the SC17(0) strain re-transformed with the RSF-Red-TER plasmid. Three of the independent transformants were investigated by using the same DNA fragment as that used for the previous experiment. The short induction time (1 hour before electroporation) was employed. KmR clones exceeding ten clones grew in each experiment. All the examined clones had the His-phenotype. In this way, a variant strain designated SC17(0) resistant to the expression of the λ Red gene was selected. This strain can be used as a recipient strain suitable for the Red-dependent integration into the Pantoea ananatis chromosome.

Reference Example 2Construction of Helper Plasmid RSF-Red-TER

The construction scheme of the helper plasmid RSF-Red-TER is shown in FIG. 2.

As a first step of the construction, a RSFsacBPlacMCS vector was designed. For this purpose, DNA fragments containing the cat gene of the pACYC184 plasmid and the structural gene region of the sacB gene of Bacillus subtilis were amplified by PCR using the oligonucleotides of SEQ ID NOS: 6 and 7, and 8 and 9, respectively. These oligonucleotides contained convenient BglII, SacI, XbaI, and BamHI restriction enzyme sites required for further cloning in the 5′ end regions, respectively. The obtained sacB fragment of 1.5 kb was cloned into the previously obtained pMW119-PlaclacI vector at the XbaI-BamHI site. This vector was constructed in the same manner as that described for the pMW118-PlaclacI vector (Skorokhodova, A. Yu et al, Biotekhnologiya (Rus), 5, 3-21 (2004)). However, this vector contained a polylinker moiety derived from pMW219 instead of the pMW218 plasmid.

Then, the aforementioned cat fragment of 1.0 kb was treated with BglII and SacI, and cloned into the RSF-PlaclacIsacB plasmid obtained in the previous step at the BamHI-SacI site. The obtained plasmid PMW-PlaclacIsacBcat contained the PlacUV5-lacI-sacB-cat fragment. In order to subclone this fragment into the RSF1010 vector, PMW-PlaclacIsacBcat was digested with BglII, blunt-ended with DNA polymerase I Klenow fragment, and successively digested with SacI. A 3.8 kb BglII-SacI fragment of the pMWPlaclacIsacBcat plasmid was eluted from 1% agarose gel, and ligated with the RSF1010 vector which had been treated with PstI and SacI. Escherichia coli TG1 was transformed with the ligation mixture, and plated on the LB medium containing chloramphenicol (50 mg/L). The plasmids isolated from the grown clones were analyzed with restriction enzymes to obtain RSFsacB. In order to construct a RSFsacBPlacMCS vector, a DNA fragment containing the PlacUV5 promoter was amplified by PCR using oligonucleotides of SEQ ID NOS: 10 and 11 as primers and the pMW119-PlaclacI plasmid as the template. The obtained fragment of 146 bp was digested with SacI and NotI, and ligated with the SacI-NotI large fragment of the RSFsacB plasmid. Then, by PCR using the oligonucleotides of SEQ ID NOS: 12 and 13 as primers, and the pKD46 plasmid (Datsenko, K A., Wanner, B. L., Proc. Natl. Acad. Sci. USA, 97, 6640-6645 (2000)) as the template, a DNA fragment of 2.3 kb containing the λRedαβγ gene and the transcription terminator tL3 was amplified. The obtained fragment was cloned into the RSFsacBPlacMCS vector at the PvuI-NotI site. In this way, the RSFRed plasmid was designed.

In order to eliminate the read through transcription of the Red gene, a ρ-dependent transcription terminator of the rrnB operon of Escherichia coli was inserted at the position between the cat gene and the PlacUV5 promoter. For this purpose, a DNA fragment containing the PlacUV5 promoter and the TrrnB terminator was amplified by PCR using the oligonucleotides of SEQ ID NOS: 14 and 11 as primers and the chromosome of Escherichia coli BW3350 as the template. These obtained fragments were treated with KpnI and ligated. Then, the 0.5 kb fragment containing both PlacUV5 and TrrnB was amplified by PCR using the oligonucleotides of SEQ ID NOS: 11 and 15 as primers. The obtained DNA fragment was digested with EcoRI, blunt-ended with DNA polymerase I Klenow fragment, digested with BamHI, and ligated with the Ecl136II-BamHI large fragment of the RSFsacBPlacMCS vector. The obtained plasmid was designated RSF-Red-TER.

The pMW118-(λattL-Kmr-λattR) plasmid was constructed based on the pMW118-attL-Tc-attR (WO2005/010175) plasmid by replacing the tetracycline resistance marker gene with the kanamycin resistance marker gene from the pUC4K plasmid (Vieira, J. and Messing, J., Gene, 19(3): 259-68 (1982)). For that purpose, the large EcoRI-HindIII fragment from pMW118-attL-Tc-attR was ligated to two fragments from the pUC4K plasmid: HindIII-PstI (676 bp) and EcoRI-HindIII (585 bp). Basic pMW118-attL-Tc-attR was obtained by ligation of the following four DNA fragments:

3) the large BglII-HindIII fragment (3916 bp) of pMW118-ter_rrnB. The plasmid pMW118-ter_rrnB was obtained by ligation of the following three DNA fragments:

a) the large DNA fragment (2359 bp) including the AatII-EcoRI fragment of pMW118 that was obtained by digesting pMW118 with EcoRI restriction endonuclease, treating with Klenow fragment of DNA polymerase I, and then digesting with AatII restriction endonuclease;

b) the small AatII-BglII fragment (1194 bp) of pUC19 including the bla gene for ampicillin resistance (ApR) was obtained by PCR amplification of the corresponding region of the pUC19 plasmid using oligonucleotides P5 and P6 (SEQ ID NOS: 59 and 60) as primers (these primers contained the subsidiary recognition sites for AatII and BglII endonucleases);

c) the small BglII-PstIpol fragment (363 bp) of the transcription terminator ter_rrnB was obtained by PCR amplification of the corresponding region of the E. coli MG1655 chromosome using oligonucleotides P7 and P8 (SEQ ID NOS: 61 and 62) as primers (these primers contained the subsidiary recognition sites for BglII and PstI endonucleases);

4) the small EcoRI-PstI fragment (1388 bp) (SEQ ID NO: 63) of pML-Tc-ter_thrL including the tetracycline resistance gene and the ter_thrL transcription terminator; the pML-Tc-ter_thrL plasmid was obtained in two steps:

the pML-ter_thrL plasmid was obtained by digesting the pML-MCS plasmid (Mashko, S. V. et al., Biotekhnologiya (in Russian), 2001, no. 5, 3-20) with the XbaI and BamHI restriction endonucleases, followed by ligation of the large fragment (3342 bp) with the XbaI-BamHI fragment (68 bp) including terminator ter_thrL obtained by PCR amplification of the corresponding region of the E. coli MG1655 chromosome using oligonucleotides P9 and P10 (SEQ ID NOS: 64 and 65) as primers (these primers contained the subsidiary recognition sites for the XbaI and BamHI endonucleases);

the pML-Tc-ter_thrL plasmid was obtained by digesting the pML-ter_thrL plasmid with the KpnI and XbaI restriction endonucleases followed by treatment with Klenow fragment of DNA polymerase I and ligation with the small EcoRI-Van91I fragment (1317 bp) of pBR322 including the tetracycline resistance gene (pBR322 was digested with EcoRI and Van91I restriction endonucleases and then treated with Klenow fragment of DNA polymerase I).

Example 1

(1) Cloning of d0191 Gene from P. ananatis SC17 Strain

By PCR using the genomic DNA of P. ananatis SC17 strain (U.S. Pat. No. 6,596,517) as the template, primers d0191 (Pa)-FW (CGCGGATCCAAGCTTTTCATTATCCAGCAGAGCG, SEQ ID NO: 16), and d0191 (Pa)-RV (CGCGGATCCTAATGCTGTAGGGCCTGAACCAG, SEQ ID NO: 17), a d0191 gene fragment containing 300 bp upstream and 200 bp downstream from the d0191 gene was obtained. Restriction enzyme BamHI sites were designed at the 5′ ends of these primers. For PCR, Pyrobest polymerase (Takara) was used, and after a reaction at 94° C. for 5 minutes, a cycle of 98° C. for 5 seconds, 55° C. for 5 seconds and 72° C. for 1 minute and 30 seconds was repeated 30 times in the standard reaction composition described in the protocol of the polymerase to amplify the target fragment of about 1.6 kb. The obtained fragment was treated with BamHI and inserted into pSTV29 (Takara) at the BamHI site (in the same direction as the lacZ gene on the vector) to obtain a plasmid pSTV-d0191F (having a chloramphenicol resistance marker) in which d0191 was cloned. The same PCR fragment was also inserted into pACYC177 (Nippon Gene) at the BamHI site (in the same direction as the kanamycin resistance gene on the vector) to obtain the plasmid pACYC-d0191F (having a kanamycin resistant marker). It was confirmed by sequencing that there was no PCR error. In this way, plasmids with two different antibiotic resistance markers and the same d0191 region (both had the same p15A origin) were prepared. Furthermore, pHSG-d0191F was prepared, which corresponds to the high copy vector pHSG299 (Takara) in which the same DNA fragment of about 1.6 kb as mentioned above was inserted at the BamHI site in the same direction as the kanamycin resistant marker and the lacZ gene on the vector.

P. ananatis SC17 was transformed with the pSTV-d0191F plasmid which was constructed as described above to prepare a d0191-enhanced strain, SC17/pSTV-d0191F. As a control strain, a strain transformed with the empty vector, SC17/pSTV29, was also prepared. Furthermore, a d0191-enhanced strain, SC17/pHSG-d0191F, was also prepared by transforming P. ananatis SC17 with the high copy pHSG-d0191F. As a control strain, a strain transformed with the empty vector, SC17/pHSG299, was also prepared. The transformation of P. ananatis was performed by a conventional method based on electroporation, and selection of the transformants was performed on LB agar medium (5 g/L of yeast extract, 10 g/L of tryptone, 10 g/L of sodium chloride, 15 g/L of agar) containing an antibiotic corresponding to the antibiotic resistance marker of the plasmid (25 mg/L in the case of chloramphenicol, 20 mg/L in the case of kanamycin).

E. coli MG1655 was transformed with the plasmid pSTV-d0191F which was constructed as described above to prepare a d0191-enhanced strain, MG1655/pSTV-d0191F. As a control strain, a strain transformed with the empty vector, MG1655/pSTV29, was also prepared. The transformation of E. coli was performed by a conventional method based on electroporation, and selection of the transformants was performed on LB agar medium (5 g/L of yeast extract, 10 g/L of tryptone, 10 g/L of sodium chloride, 15 g/L of agar) containing an antibiotic corresponding to the antibiotic resistance marker of the plasmid (25 mg/L in the case of chloramphenicol, 20 mg/L in the case of kanamycin).

Deletion of the d0191 gene was performed by “Red-driven integration”, first developed by Datsenko and Wanner (Proc. Natl. Acad. Sci. USA, 2000, vol. 97, No. 12, pp. 6640-6645). According to the “Red-driven integration” method, a gene-disrupted strain can be constructed in one step using a PCR product obtained with synthetic primers in which a part of a target gene is present on the 5′ side, and a part of antibiotic resistance gene is present on the 3′ side, respectively DNA fragment with sequences homologous to each end of the d0191 gene flanking an antibiotic resistance gene (kanamycin resistance gene) was obtained by PCR. The primers Dd0191-FW (CCGTGTCTGAAGCCTATTTTGCCCGCCTGCTGGGCTTGCCTTTTATTGCCTGAAGCCT GCTTTTTTATACTAAGTTGGCA, SEQ ID NO: 18), and Dd0191-RV (CTAGCCCAGTTCGCGCTGCCAGGGCGTAATATCGCCAATGTGCTCGGCAACGCTCA AGTTAGTATAAAAAAGCTGAACGA, SEQ ID NO: 19) were used, and pME118-(λ attL-Kmr-λattR) (WO2006/093322A2, SEQ ID NO: 18) was used as the template.

For PCR, Pyrobest polymerase (Takara) was used, and after a reaction at 94° C. for 5 minutes, a cycle of 98° C. for 5 seconds, 55° C. for 5 seconds and 72° C. for 2 minute and 30 seconds was repeated 30 times in the standard reaction composition described in the protocol of the polymerase to amplify the target DNA fragment. The obtained DNA fragment (DNA fragment with the kanamycin resistant marker and homologous sequences of d0191 on both sides of the marker, FIG. 3) was introduced into P. ananatis SC17(0)/RSF-Red-TER strain by electroporation to obtain a kanamycin-resistant strain. It was confirmed that the kanamycin-resistant strain contained a d0191 gene which had been disrupted by insertion of the kanamycin-resistant cassette into the d0191 gene on the chromosome with the homologous sequences of the d0191 gene region present at both ends of the DNA fragment (SC17(0) d0191::Kmr strain, FIG. 3).

Then, by introducing the chromosomal DNA prepared from this SC17(0) d0191::Kmr strain into the SC17 strain by electroporation, a SC17 d0191::Kmr strain, which was a d0191 gene-disrupted strain derived from the SC17 strain, was finally obtained.

It is known that the pMIV-CysE5 plasmid includes the cysE5 gene encoding a mutant SAT which is desensitized to feedback inhibition (U.S. Patent Published Application No. 20050112731(A1)), and a cysteine-producing bacterium which produces a marked amount of cysteine can be prepared by introducing this plasmid (U.S. Patent Published Application No. 20050112731(A1), U.S. Pat. No. 5,972,663 etc.). The construction method of pMIV-CysE5 is described below.

The mutant allele E. coli cysE gene, cysE5 (U.S. Patent Published Application No. 20050112731(A1)) was obtained by PCR using a cysEplF primer (5′-agc-tga-gtc-gac-atg-tcg-tgt-gaa-gaa-ctg-gaa-3′, SEQ ID NO: 20), a cysER primer (5′-agc-tga-tct-aga-ata-gat-gat-tac-atc-gca-tcc-3′, SEQ ID NO: 21) and the template pMW-PompC-cysE5 (EP1650296A1) (a cycle of 94° C. for 0.5 minute, 57° C. for 0.5 minute and 72° C. for 1 minute was repeated 27 times, and then the reaction was allowed to stand at 72° C. for 7 minutes). The cysEplF primer was designed so as to bind with the start codon ATG and a downstream sequence of the E. coli cysE gene, and had a 6-mer SalI site at the 5′ end. The cysER primer was designed so as to bind with the stop codon and an upstream sequence of the E. coli cysE gene, and had a 6-mer XbaI site at the 5′ end. A DNA fragment of about 0.7 kb obtained by PCR was digested with SalI and XbaI, and the digestion product was inserted into pMIV-PompC which had been similarly digested with SalI and XbaI to construct pMIV-CysE5.

The pMIV-PompC plasmid described above was constructed as follows. By PCR using the genomic DNA of E. coli MG1655 strain as the template, a PrOMPCF primer (5′-agc-tga-gtc-gac-aac-cct-ctg-tta-tat-gcc-ttt-a-3′, SEQ ID NO: 22), and a PrOMPCR primer (5′-agc-tga-gca-tgc-gag-tga-agg-ttt-tgt-ttt-gac-3′, SEQ ID NO: 23), a DNA fragment containing about 0.3 kb of the promoter region of the ompC gene was obtained, and this fragment was inserted into pMIV-5JS at the PaeI and SalI sites to construct pMIV-PompC. The pMIV-5JS plasmid was constructed by ligating the BamHI and HindIII sites designed beforehand at both ends of the intJS cassette (described later) with the BglII and HindIII sites of pM12-ter(thr) (described later) (FIG. 4).

(c) By PCR using an upstream primer (cagctgcagt ctgttacagg tcactaatac c, SEQ ID NO: 30, designed to have PstI site), a downstream primer (ccgagctccg ctcaagttag tataaaaaag ctgaacg, SEQ ID NO: 31, designed to have SacI site), and pMW118-attL-tet-attR-ter_rrnB (WO2005/010175) as the template, an LattR fragment of 0.16 kbp was obtained.

(d) By ligation of the LattL fragment and the CmR fragment, a LattL-CmR fragment of 1.15 kbp was obtained.

(e) By ligation of the LattL-CmR fragment and the LattR fragment digested with PstI, a LattL-CmR-LattR fragment of 1.31 kbp was obtained.

(f) By annealing a synthetic oligonucleotide (cccgagctcg gtacctcgcg aatgcatcta gatgggcccg tcgactgcag aggcctgcat gcaagcttcc, SEQ ID NO: 32) and a synthetic oligonucleotide which is a complementary strand of the former, a double strand DNA fragment of 70 bp constituting a multi-cloning site (MCS) was obtained.

(g) By ligation of the LattL-CmR-LattR fragment and the double strand DNA fragment constituting a multi-cloning site (MCS), both digested with SacI, an IntJS fragment of 1.38 kbp was obtained.

The plasmid pACYC-d0191F containing the d0191 gene (kanamycin resistant) constructed as described above was introduced into the SC17 strain (SC17/pACYC-d0191F), and the an inhibition-desensitized SAT gene-carrying plasmid pMIV-CysE5 (chloramphenicol resistant) was further introduced into the strain to construct a P. ananatis strain having cysteine-producing ability and enhanced d0191 (SC17/pMIV-CysE5/pACYC-d0191F strain). Furthermore, as a control strain, SC17/pMIV-CysE5/pACYC177 was prepared, which was transformed with the empty vector pACYC177 instead of pACYC-d0191F.

The d0191-deficient strain, SC17 d0191::Kmr strain, constructed as described above was transformed with the pMIV-CysE5 plasmid to prepare a d0191-deficient cysteine-producing bacterium, SC17 d0191::Kmr/pMIV-CysE5. As a control strain, an SC17/pMIV-CysE5 strain was prepared, which corresponded to the SC17 strain introduced with pMIV-CysE5.

(8) Effect of Deletion of d0191 and Enhancement of d0191 on Cysteine Resistance in P. ananatis SC17 Strain

In order to investigate influence of the d0191 gene on cysteine resistance, the d0191-deficient strain, SC17 d0191::Kmr strain, and the d0191-amplified strain, SC17/pSTV-d0191F strain, as well as the respective control strains, the SC17 strain and the SC17/pSTV29 strain, respectively, were each cultured in M9 minimal medium (Sambrook et al., Molecular Cloning, 3rd edition, 2001, Cold Spring Harbor Laboratory Press) containing cysteine of different concentrations, and the difference in cysteine resistance was evaluated by determining any increase in growth. In this culture, as the resistance to cysteine increased, the OD of the medium increased more quickly, and as the resistance to cysteine decreased, the OD of the medium increased at a much slower rate. The procedure of the experiment was as follows. Each strain was precultured overnight in M9 minimal medium containing 0.4% glucose (not containing cysteine) (3-ml test tube, 34° C., shaking culture), and then inoculated into the main culture medium. At the time of the inoculation, the OD of the preculture was measured, and the inoculation amount was adjusted so that the main culture is started with the same amounts of cells. The OD at the time of the start of the main culture was about 0.007.

The main culture was performed in 4 ml of the M9 minimal medium containing cysteine at one of various concentrations (0 mM, 1 mM, 5 mM) and 0.4% glucose using an automatically OD measuring culture apparatus, BIO-PHOTORECORDER TN-1506 (ADVANTEC) and the L-shaped test tube for the apparatus. 25 mg/L of chloramphenicol was added to the medium for the strains containing the plasmid. Progress of the culture (growth curves) are shown in FIG. 8. There was observed a tendency that as the concentration of cysteine increased, the increase of the OD was slower. However, the d0191-enhanced strain, SC17/pSTV-d0191F strain grew more quickly as compared to the control SC17/pSTV29 strain, and it was found that it was imparted with cysteine resistance. Furthermore, since growth of the d0191-deficient strain, SC17 d0191::Kmr, was completely inhibited by addition of cysteine, it was found that the cysteine resistance thereof decreased.

(9) Effect of Enhancing d0191 on Cysteine Production in E. coli MG1655 Strain

In order to investigate effect of enhancing the d0191 gene on cysteine resistance, the cysteine-producing abilities of the d0191-amplified strain, MG1655/pSTV-d0191F, a control strain thereof, MG1655/pSTV29, were each cultured in the M9 minimal medium containing cysteine at different concentrations, and difference in cysteine resistance was evaluated by determining rising of the growth. The culture was performed basically according to the method described in (8), except that the OD at the start of the main culture was about 0.006, the culture temperature was 37° C., and the cysteine concentrations were 0 mM, 0.1 mM, 0.2 mM. Progress of the culture (growth curves) is shown in FIG. 9. There was observed a tendency that as the concentration of cysteine increased, the increase in the OD was slower. However, the d0191-enhanced strain, MG1655/pSTV-d0191F strain grew more quickly compared with the control MG1655/pSTV29 strain, and it was found that it was imparted with cysteine resistance. From the above results, it was found that when d0191 was enhanced, cysteine resistance was enhanced regardless of whether the host is P. ananatis or E. coli.

The phenotype concerning cysteine resistance suggested a possibility of involvement of the d0191 product in cysteine decomposition. Therefore, in order to examine whether the d0191 gene product had cysteine desulfhydrase activity (henceforth abbreviated as the “CD activity”) indicative of cysteine decomposition, a d0191-deficient strain, a d0191-enhanced strain and their respective control strains were cultured, proteins in cell extracts of them were separated by native PAGE, and their CD activity was detected by activity staining. The methods of native PAGE and activity staining were according to the descriptions of N. Awano et al. (Effect of cysteine desulfhydrase gene disruption on L-cysteine overproduction in Escherichia coli, Appl. Microbiol. Biotechnol., 2003 August; 62(2-3):239-43).

First, the four strains, the d0191-deficient strain SC17 d0191::Kmr strain constructed as described above and its control, the SC17 strain, as well as the d0191-enhanced strain SC17/pHSG-d0191F strain and its control, SC17/pHSG299, were cultured by the method described below, and cell extracts of each were prepared. An overnight culture in LB medium (5 g/L of yeast extract, 10 g/L of tryptone, 10 g/L of sodium chloride) for each strain was inoculated into 50 ml of LB medium contained in a Sakaguchi flask so as to be diluted 100 times, and culture was performed with shaking. After 3 hours, cells in the logarithmic phase were collected from the medium (OD was around 0.5), washed with a washing buffer (10 mM Tris-HCl (pH 8.6), 0.1 mM DTT (dithiothreitol), 0.01 mM PLP (pyridoxal-5′-phosphate)), then suspended in 1 ml of the washing buffer, and disrupted by ultrasonication, and the supernatant (cell extract) was obtained by centrifugation. The culture was performed at 34° C. for all the strains, and 20 mg/L of kanamycin was added to the medium for the strains containing a plasmid. Protein concentration of each cell extract prepared as described above was quantified, and the extract was diluted to a protein concentration of 2.5 mg/ml with the washing buffer, and 5× loading buffer was added (10 mM Tris-HCl (pH 8.6), 30% glycerol, 0.005% BPB (Bromophenol Blue)) to prepare a sample at a final concentration of 2 mg/ml.

One microliter (2 μg of protein) of each sample was loaded onto a 10% native PAGE gel (TEFCO), and native PAGE was performed at 4° C. and 20 mA for 1.5 hours. The composition of the gel-running buffer used in the native PAGE consisted of 25 mM Tris-HCl (pH 8.3) and 192 mM glycine. After the electrophoresis, the gel was immersed in a staining solution (100 mM Tris-HCl (pH 8.6), 10 mM EDTA (ethylenediaminetetraacetic acid), 50 mM L-cysteine, 0.02 mM PLP, 1.6 mM bismuth(III) chloride), and activity staining was performed at room temperature by slowly shaking the gel until the appropriate coloring was observed. The detected bands of the d0191 product presented by the CD activity are shown in FIG. 10. A band not seen with the d0191-deficient strain was observed with the control strain, and increased intensity of this band was observed with the d0191-enhanced strain. From these results, it was shown that a band of the d0191 product stained by the CD activity. From the above results, it was revealed that d0191 coded for a novel cysteine desulfhydrase, and was involved in cysteine resistance.

(11) Effect of d0191 Enhancement on Cysteine Production in Cysteine-Producing Bacterium

The culture was performed according to the following procedures. The SC17/pMIV-CysE5/pACYC-d0191F and the SC17/pMIV-CysE5/pACYC177 strains were each applied and spread onto LB agar medium containing chloramphenicol and kanamycin, and precultured overnight at 34° C. The cells corresponding to about 7 cm on the plate were scraped with an inoculating loop of 10 μl size (NUNC Blue Loop), and inoculated into 2 ml of the cysteine production medium in a large test tube (internal diameter: 23 mm, length: 20 cm). The amounts of inoculated cells were adjusted so that the cell amounts at the time of the start of the culture were substantially the same. The culture was performed at 32° C. with shaking. For both strains, when glucose was completely consumed, the culture was ended (about 22 to 25 hours), and the amount of cysteine which accumulated in the medium was quantified. The quantification of cysteine was performed by the method described by Gaitonde, M. K. (Biochem. J., 1967 Aug., 104(2):627-33). The experiment was performed in tetraplicate for the both strains, and averages and standard deviations of the accumulated cysteine amounts are shown in Table 1. As shown in Table 1, it was revealed that enhancement of d0191 had the effect of decreasing cysteine accumulation amount.

Since it became clear that d0191 participates in decomposition of cysteine, and enhancing the gene reduced cysteine production, it was examined whether suppression of the activity of d0191 (specifically, deficiency of the gene) had a positive effect on the cysteine production. In order to examine whether the d0191-deficient cysteine-producing strain, SC17 d0191::Kmr/pMIV-CysE5, prepared as described above had superior cysteine production ability as compared to the d0191-non-disrupted strain, SC17/pMIV-CysE5, they were cultured for cysteine production, and the abilities thereof were compared. The methods for the cysteine production culture and quantification of cysteine were the same as those described in (11), provided that, as for the antibiotics, only chloramphenicol was added to the medium. The experiment was performed in tetraplicate for the control strain, and in octaplicate for the d0191-deficient strain. Averages and standard deviations of the accumulated cysteine amounts are shown in Table 2. As shown in Table 2, it was revealed that suppression of the activity of d0191 had an effect of increasing accumulation amount of cysteine.

While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. Each of the aforementioned documents is incorporated by reference herein in its entirety.

What is claimed is:1. A bacterium belonging to the family Enterobacteriaceae, which has L-cysteine-producing ability and has been modified to decrease the activity of a protein selected from the group consisting of:
(A) a protein comprising the amino acid sequence of SEQ ID NO: 2, and(B) a protein comprising the amino acid sequence of SEQ ID NO: 2 but which includes substitutions, deletions, insertions, or additions of one or several amino acid residues, and wherein said protein comprises cysteine desulfhydrase activity.2. The bacterium according to claim 1, wherein the activity of the protein is decreased by a method selected from the group consisting of decreasing the expression of a gene encoding the protein, and by disrupting the gene encoding the protein.3. The bacterium according to claim 1, wherein the gene is selected from the group consisting of:
(a) a DNA comprising the nucleotide sequence of SEQ ID NO: 1, and(b) a DNA which is able to hybridize with a sequence complementary to the nucleotide sequence of SEQ ID NO: 1, or a probe prepared from the nucleotide sequence, under stringent conditions, and encodes a protein comprising cysteine desulfhydrase activity.4. The bacterium according to claim 1, which further has a serine acetyltransferase which has been mutated so that feedback inhibition by L-cysteine is attenuated.5. The bacterium according to claim 1, which is a Pantoea bacterium.6. The bacterium according to claim 5, which is Pantoea ananatis. 7. A method for producing and L-amino acid selected from the group consisting of L-cysteine, L-cystine, derivatives thereof, and combinations thereof, which comprises culturing the bacterium according to claim 1 in a medium and collecting the L-amino acid from the medium.8. The method according to claim 7, wherein the derivative of L-cysteine is a thiazolidine derivative.9. A DNA encoding the protein selected from the group consisting of:
(A) a protein comprising the amino acid sequence of SEQ ID NO: 2, and(B) a protein comprising the amino acid sequence of SEQ ID NO: 2 but which includes substitutions, deletions, insertions or additions of 1 or several amino acid residues, and wherein said protein comprises cysteine desulfhydrase activity.10. The DNA according to claim 9, which is a DNA selected from the group consisting of:
(a) a DNA comprising the nucleotide sequence of SEQ ID NO: 1, and(b) a DNA which is able to hybridize with a sequence complementary to the nucleotide sequence of SEQ ID NO: 1, or a probe prepared from the nucleotide sequence, under stringent conditions, and encodes a protein having cysteine desulfhydrase activity.

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